The present invention relates generally to heat exchangers, and more specifically to heat exchangers of the type used in refrigerated water coolers or the like for extracting heat from incoming drinking water by means of an evaporative refrigerant.
In the past, heat exchangers used in water coolers were generally of three types, namely the instantaneous type, the semi-instantaneous type, and the storage tank type. The instantaneous type heat exchanger comprises generally two fluid-conducting tubes disposed in intimate thermal contact with one another. One tube carries a low boiling point refrigerant, such as Freon, while the other tube conducts the water to be cooled. Instantaneous heat exchangers of this type, while being comparatively efficient for continuous flow applications, are not particularly suited for use in drinking water coolers because of their limited storage capacity and comparatively low thermal efficiency for intermittent flow applications.
Prior art semi-instantaneous-type exchangers typically comprise a relatively small volume metal receptacle or reservoir having a refrigerant evaporator (cooling) coil helically wrapped around and disposed in good thermal contact with its outer side wall, and a helical water-conducting coil bonded in good thermal contact with either the wall of the receptacle or the cooling coil and arranged to circulate influent drinking water therethrough and discharge the chilled water into the receptacle or reservoir. However, since the chilled water from the water-conducting coil is delivered to the lower portion of the receptacle and since the cooled water is drawn from the bottom of the receptacle upon opening of the bubbler valve or faucet of the water cooler, the main body of water which is stored in the receptacle remains substantially uncirculated and tends simply to act as a thermal flywheel. Thus, while the semi-instantaneous type heat exchanger is generally considered to be more efficient for use in refrigerated water coolers than the instantaneous type, it is generally larger, more expensive and requires more elaborate controls to prevent freeze-ups. The storage tank-type heat exchanger generally comprises a relatively larger volume metal tank or receptacle having a refrigerant evaporator or cooling coil wound around its outer side wall so as to cool the tank and water contained therein by conduction. Baffle means may be incorporated in the storage tank to promote thermal stratification of the water within the tank, thus keeping the coolest water toward the bottom of the tank from whence it is withdrawn upon opening of the bubbler valve or faucet of the water cooler. The storage tank-type heat exchanger has the advantages of a relatively larger storage capacity and, hence, the capability of providing a larger volume of chilled water for a short time interval, and also the ability to tolerate controlled ice formation along the inner surface of the tank. However, the storage tank-type heat exchanger is relatively bulky and has a comparatively low heat transfer coefficient under flow conditions due to the unconfined flow of water over the chilled surfaces of the tank. These characteristics tend to compromise the rated capacity of the water cooler for a given cabinet size.